With a seriously lagging stock price and a number of possible merger ideas on the table, AT&T boss Michael Armstrong has plenty of headaches.

But one of his biggest is coming from an unlikely source: Time Warner’s Gerry Levin.

And it could prove to be a $15 billion headache.

After AT&T’s merger with MediaOne, regulators demanded that Armstrong divest some assets in exchange for approval.

AT&T was given the options of selling off millions of cable customers, cutting ties with John Malone’s Liberty Media or bailing out of its 25 percent stake in Time Warner Entertainment, which houses Time Warner’s cable assets. AT&T has until December to choose, and sources say Armstrong wants out of Time Warner Entertainment.

But none of the other media giants want to take it off his hands and get in bed with Time Warner and AOL. For most of the other major players, such an entanglement with a direct competitor would cause regulatory problems they don’t need – and not all are convinced the AOL-Time Warner powerhouse will be an effective business.

That leaves Armstrong with only one buyer for the 25 percent stake: Time Warner, itself.

Sources said Armstrong approached Levin nearly a year ago with a price he thought was respectable: $20 billion.

But Levin, realizing that he was negotiating from a position of power, had a much smaller number in mind: $5 billion.

“Armstrong told Levin he thought $20 billion was a fair price, and Levin said, ‘Hmmm, how about $5 billion,'” said a source familiar with the situation.

“And nothing has changed in nine months. They are at total loggerheads.”

As the months slip by and AT&T’s stock remains in the dumps, Levin has less and less reason to buy the stake for anything but a bargain price.

“Gerry knows Armstrong’s on the ropes, so he’s just dug in his heels. He knows Armstrong has nowhere else to go,” said the source.

A Time Warner spokesman said the company is “discussing a lot of issues with AT&T,” and the “talks are ongoing.” AT&T declined to comment.

Armstrong may have gotten a little help on the issue recently from an unlikely source: the feds.

Time Warner’s relationship with AT&T has also reportedly come under scrutiny from the government lawyers looking at the AOL-Time Warner deal.

While that may give Levin added incentive to get a deal done on the stake, he’s still in the catbird seat, and he knows it.

Armstrong has floated a raft of ideas about how to turn around his stock price, which has dropped from a high of $60 in April to $30.25 on Friday.

Nearly every operating unit of AT&T has been considered as a possible spin-off, and there reportedly have been informal merger discussions with British Telecommunications.

AT&T Wireless, whose tracking stock has fallen from its offering price of $29.50 to $25.75, is looking at a deal with Nextel.